Image Sensor Isolation Region and Method of Forming the Same

ABSTRACT

Image sensors comprising an isolation region according to embodiments are disclosed, as well as methods of forming the image sensors with isolation region. An embodiment is a structure comprising a semiconductor substrate, a photo element in the semiconductor substrate, and an isolation region in the semiconductor substrate. The isolation region is proximate the photo element and comprises a dielectric material and an epitaxial region. The epitaxial region is disposed between the semiconductor substrate and the dielectric material.

This application is a continuation of U.S. patent application Ser. No. 13/415,546, filed on Mar. 8, 2012, entitled “Image Sensor Isolation Region and Method of Forming the Same,” which application is hereby incorporated herein in its entirety by reference.

BACKGROUND

As technologies evolve, complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, a CMOS image sensor may have a high image acquisition rate, a lower operating voltage, lower power consumption and higher noise immunity. In addition, CMOS image sensors may be fabricated on the same high volume wafer processing lines as logic and memory devices. As a result, a CMOS image chip may comprise both image sensors and any necessary logic, such as amplifiers, A/D converters, or the like.

CMOS image sensors generally are pixelated metal oxide semiconductors. A CMOS image sensor typically comprises an array of light sensitive picture elements (pixels), each of which may include transistors, capacitors, and a photo-sensitive element. A CMOS image sensor utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo diode formed in a substrate. As the photo diode is exposed to light, an electrical charge is induced in the photo diode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. Furthermore, the electrons are converted into a voltage signal in the pixel and further transformed into a digital signal by means of an A/D converter. A plurality of periphery circuits may receive the digital signals and process them to display an image of the subject scene.

A CMOS image sensor may comprise a plurality of additional layers, such as dielectric layers and interconnect metal layers, formed on top of the substrate, wherein the interconnect layers are used to couple the photo diode with peripheral circuitry. The side having additional layers of the CMOS image sensor is commonly referred to as a front side, while the side having the substrate is referred to as a backside. Depending on the light path difference, CMOS image sensors can be further divided into two major categories, namely front side illuminated (FSI) image sensors and backside illuminated (BSI) image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross sectional views of a method of forming a backside illuminated image sensor in accordance with an embodiment.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the embodiments of the disclosure, and do not limit the scope of the disclosure.

The present disclosure describes embodiments in a specific context, namely a backside illuminated image sensor. Other embodiments may also be applied, however, to a variety of image sensors, such as a front side illuminated image sensor or the like. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings. Although various steps of a disclosed method may be discussed in a particular order, other methods may be performed in any logical order.

FIG. 1 illustrates a cross sectional view of a substrate 10 of a pixel of an image sensor during processing. The substrate 10 includes a lightly p-type doped epitaxial layer over a highly p-type doped silicon substrate, for example. In other embodiments, the substrate 10 may include silicon, germanium, silicon germanium, graded silicon germanium, a bulk semiconductor material, a semiconductor-on-insulator, carbon, quartz, sapphire, glass, or the like, and may be multi-layered (e.g., strained layers). A deep p-well (DPW) region 12 is formed in the substrate 10, and a cell p-well (CPW) region 14 is formed over the DPW region 12 in the substrate 10. The DPW region 12 and the CPW region 14 are components of an isolation region formed in the substrate 10 to prevent cross walk between adjacent pixels. The DPW region 12 and CPW region 14 are formed by, for example, implanting p-type dopants, such as boron or the like, through a front side 13 of the substrate 10. In an embodiment, a p-type doping concentration of the CPW region 14 is greater than a p-type doping concentration of the DPW region 12. The DPW region 12 can have a doping concentration in a range from about 10¹³/cm³ to about 10¹⁸/cm³. The CPW region 14 can have a doping concentration in a range from about 10¹⁵/cm³ to about 10¹⁹/cm³.

FIGS. 2 through 4 are cross sectional views of the CPW region 14 in which a trench isolation is formed according to an embodiment. In FIG. 2, a trench 16 is etched in the CPW region 14 in the substrate 10. The etch may be performed using acceptable photolithography techniques. Although not explicitly illustrated, a mask can be used during the photolithography process and used during a subsequent epitaxial growth to prevent nucleation outside of the trench 16.

In FIG. 3, an epitaxial layer 18 (including first portion 18 ₁ and second portion 18 ₂) is epitaxially grown on the surfaces of the substrate 10 in the trench 16. In an embodiment, the epitaxial layer 18 is silicon germanium (SiGe) layer, such as Si_(0.99-0.98)Ge_(0.01-0.02). In this embodiment where the substrate 10 is silicon, including epitaxial silicon, the SiGe layer can induce a stress, such as a compressive stress, in the substrate 10. Accordingly, the epitaxial layer 18 can be lattice mismatched with the substrate 10 and can induce a stress, such as compressive or tensile, in the substrate 10. Other acceptable materials can be used for the epitaxial layer 18. The epitaxial layer 18 can be deposited by appropriate epitaxial growth processes, such as selective epitaxial growth (SEG), molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), the like, or a combination thereof.

In an embodiment, a first portion 18 ₁ of the epitaxial layer 18 is grown from the substrate 10 without doping, and a second portion 18 ₂ of the epitaxial layer 18 is grown from the first portion 18 ₁ with in situ doping during the growth. In an embodiment, the dopant is a p-type dopant, such as boron or the like, although a different dopant type, such as n-type can be used for other applications. More specifically, in an embodiment, the first portion 18 ₁ of the epitaxial layer 18 is SiGe, and the second portion 18 ₂ of the epitaxial layer 18 is SiGe doped with boron. In these embodiments, the epitaxial growth can begin without a dopant for the growth of the first portion, and the epitaxial growth can be later changed to include a dopant for the growth of the second portion. In other embodiments, the entirety of the epitaxial layer 18 can be doped, with a p-type or n-type dopant, or none of the epitaxial layer 18 can be doped.

The epitaxial layer 18 can repair damage to the substrate 10 that can be caused by the trench etch. By repairing the damage to the substrate 10, detrimental leakage current that could otherwise occur by the damage is generally reduced.

In FIG. 4, a dielectric material 20, such as an oxide, is deposited in the trench 16 over the epitaxial layer 18. The dielectric material 20 can be deposited by a high density plasma chemical vapor deposition (HDP-CVD), plasma enhanced chemical vapor deposition (PECVD), a thermal CVD, the like, or a combination thereof. Further, the dielectric material 20 can be any dielectric material formed by an acceptable process. A planarization, such as by a chemical mechanical polish (CMP), may be performed on the front side after the deposition of the dielectric material 20. FIG. 5 illustrates the formed isolation regions, including the DPW region 12, CPW region 14, epitaxial layer 18, and dielectric material 20 formed in the substrate 10 of FIG. 1.

FIG. 6 is a cross sectional view of the substrate 10 after further processing according to an embodiment. A photo diode is formed in the substrate 10. The photo diode comprises an n-type region 22 and a p-type region 24 that form a p-n junction. The n-type region 22 has an n-type dopant concentration of, for example, between about 10¹⁵/cm³ and about 10¹⁹/cm³, and the p-type region 24 has a p-type dopant concentration of, for example, between about 10¹⁶/cm³ and about 10²⁰/cm³. The photo diode is formed proximate one of the isolation regions. A source/drain region 26 is formed in the substrate 10 in the CPW region 14 of the other illustrated isolation region. The source/drain region 26 in this example is an n-type region having an n-type dopant concentration of, for example, between about 10¹⁶/cm³ and about 10²⁰/cm³. An extension region 28 extends from the source/drain region 26, and has an n-type dopant concentration of, for example, between about 10¹⁸/cm³ and about 10²²/cm³. These regions 22, 24, 26, and 28 can be formed by appropriate implantation through the front side 13.

A gate dielectric layer 34 is formed on the front side 13 over the substrate 10. A gate electrode layer, such as polysilicon or the like, is deposited over the gate dielectric layer 34 and patterned into a gate electrode 30. The gate electrode 30 comprises an n-type doped upper region 32 formed by implantation, in situ doping during deposition, or the like, and has an n-type dopant concentration, for example, between about 10⁷/cm³ and about 10¹⁴/cm³. The gate electrode 30 is laterally between the photo diode and the source/drain region 26, and partially over the extension region 28. A dielectric layer 36, such as tetraethyl orthosilicate (TEOS) or the like, is conformally deposited over the substrate 10 and the gate electrode 30, and a resist protective layer (RPL) 38, such as silicon oxide or the like, is conformally deposited over the dielectric layer 36. The dielectric layer 36 and RPL 38 can be etched to expose a top surface of the gate electrode 30 and are recessed below the top surface of the gate electrode 30. An etch stop layer (ESL) 40, such as silicon nitride or the like, is conformally deposited over the RPL 38. An inter-layer dielectric (ILD) 42, such as silicon oxide, borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), fluorinated silicate glass (FSG), carbon-doped silicon oxide, or the like, is deposited over the ESL 40. The ILD 42 can be planarized using, for example, a CMP. These components can be formed with any acceptable deposition process.

Contacts (not explicitly illustrated) can be formed through the ILD 42 to the substrate 10. For example, openings can be etched using acceptable photolithography techniques, and conductive materials, such as a copper, aluminum, or the like, with or without a barrier layer, can be deposited into the openings. A polishing and/or grinding process, such as a CMP, can remove excess conductive materials and leave the contacts in the openings.

As shown in the cross sectional view of FIG. 7, subsequent inter-metal dielectrics (IMDs) 44, 46, 48, and 50 having metallization patterns 54 and vias 52 are formed over the ILD 42. Each of the IMDs 44, 46, 48, and 50 can be silicon oxide, BPSG, PSG, FSG, the like, or a combination thereof, formed by CVD, HDP-CVD, furnace deposition, PECVD, the like, or a combination thereof. The vias 52 and metallization patterns 54 in each of the IMDs 44, 46, 48, and 50 can be formed using, for example, a damascene process like a dual damascene. Conductive materials, such as aluminum, copper, or the like, with or without a barrier layer, can be used as the vias 52 and metallization patterns 54 and can be deposited using, for example, CVD, atomic layer deposition (ALD), physical vapor deposition (PVD), the like, or a combination thereof. A polishing and/or grinding process, such as a CMP, can remove excess conductive materials. The IMDs 44, 46, 48, and 50 are sequentially formed, along with the respective vias 52 and metallization patterns 54.

A first passivation layer 56 is formed over the top IMD 50, and a second passivation layer 58 is formed over the first passivation layer 56. The passivation layers 56 and 58 can be silicon nitride, silicon carbide, silicon oxide, the like, or a combination thereof, deposited by CVD or the like. It should be noted that many other components may be included in an embodiment that are not expressly depicted. For example, etch stop layers can be disposed between the various interfaces between layers of the ILD 42 and IMDs 44, 46, 48, and 50. Further, more or fewer IMDs can be used.

FIG. 8 is a cross sectional view after the substrate 10 is flipped and bonded on a carrier 60. Once the passivation layers 56 and 58 are formed, the substrate 10 is flipped and bonded on the carrier 60, which may be silicon, glass, or the like. In particular, the front side 13 of the substrate 10 faces toward the carrier 60. Various bonding techniques may be employed to achieve bonding between front side 13 of the substrate 10, e.g., the second passivation layer 58, and the carrier 60. Suitable bonding techniques may include adhesive bonding, vacuum bonding, anodic bonding, or the like. The carrier 60 may provide sufficient mechanical support to resist forces due to a grinding step of a thinning process.

A thinning process is performed on a backside of the substrate 10. The substrate 10 is thinned until the lightly doped p-type epitaxial layer of the substrate 10 is exposed from the backside 65. A thin substrate 10 allows more light to pass through the substrate 10 and hit photo diodes embedded in the substrate 10 without being absorbed by the substrate 10. The thinning process may be implemented by using suitable techniques, such as grinding, polishing and/or chemical etching.

Deep n-type regions 62 and 64 are formed in the substrate 10 by an ion implantation into the substrate 10 through the backside 65 of the substrate 10. The deep n-type region 62 is disposed between the deep n-type region 64 and the n-type region 22 of the photo diode. The deep n-type region 62 can have a doping concentration in a range from about 10¹⁵/cm³ to about 10¹⁸/cm³. The deep n-type region 64 can have a doping concentration in a range from about 10¹⁴/cm³ to about 10¹⁷/cm³. As a result, an extended photo active region is formed.

A thin p+ layer 66 is formed on the backside 65 of the substrate 10. The thin p+ layer is formed by an ion implantation through the backside 65 of the substrate 10, and has a p-type dopant concentration from about 10¹⁷/cm³ to about 10²²/cm³. The thin p+ layer 66 may be formed on the backside 65 of the substrate 10 to increase the number of photons converted into electrons. In order to repair crystal defects that can be caused by ion implantation and to activate the implanted ions, a laser annealing process may be performed on the backside of the substrate 10.

A dielectric layer 68 is formed over the thin p+ layer 66. The dielectric layer 68 can be a silicon nitride, silicon oxide, silicon oxynitride, silicon oxycarbide, a high-k dielectric such as dielectrics with a dielectric constant greater than 2, the like, or a combination thereof, deposited by CVD, metalorganic chemical vapor deposition (MOCVD), ALD, the like, or a combination thereof. A color filter layer 70 is formed over the dielectric layer 68. The color filter layer 70 may be used to allow specific wavelengths of light to pass while reflecting other wavelengths, thereby allowing the image sensor to determine the color of the light being received by the photo diode. The color filter layer 70 may vary, such as a red, green, and blue filter. Other combinations, such as cyan, yellow, and magenta, may also be used. The number of different colors of the color filter layer 70 may also vary. The color filter layer 70 may comprise a pigmented or dyed material, such as an acrylic. For example, polymethyl-methacrylate (PMMA) or polyglycidylmethacrylate (PGMS) are suitable materials with which a pigment or dye may be added to form the color filter layer 70. Other materials, however, may be used. The color filter layer 70 may be formed by any suitable method.

A microlens layer 72 is formed over the color filter layer 70. The microlens layer 72 may be formed of any material that may be patterned and formed into lenses, such as a high transmittance, acrylic polymer. In an embodiment, the microlens layer 72 may be formed using a material in a liquid state and spin-on techniques. This method has been found to produce a substantially planar surface and a microlens layer 72 having a substantially uniform thickness, thereby providing greater uniformity in the microlenses. Other methods, such as deposition techniques like CVD, PVD, or the like, may also be used.

An embodiment is a structure comprising a semiconductor substrate, a photo element in the semiconductor substrate, and an isolation region in the semiconductor substrate. The isolation region is proximate the photo element and comprises a dielectric material and an epitaxial region. The epitaxial region is disposed between the semiconductor substrate and the dielectric material.

Another embodiment is a structure comprising a photo diode in a substrate, an epitaxial layer on a surface of a trench in the substrate, and an isolation material on the epitaxial layer. The trench is proximate the photo diode. The epitaxial layer is disposed between the isolation material and the substrate.

A further embodiment is a method comprising forming a recess in a substrate; epitaxially growing a regrowth layer on a surface of the recess; forming an isolation material on the regrowth layer; and forming a photo element in the substrate.

Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method comprising: forming a recess in a substrate; epitaxially growing a regrowth layer on a surface of the recess; forming an isolation material on the regrowth layer; and forming a photo element in the substrate.
 2. The method of claim 1 further comprising doping the regrowth layer with a p-type or n-type dopant.
 3. The method of claim 1, wherein the epitaxially growing the regrowth layer comprises growing a first portion of the regrowth layer without a dopant and subsequently growing a second portion of the regrowth layer with the dopant.
 4. The method of claim 1, wherein the regrowth layer comprises a material that is lattice mismatched to the substrate.
 5. The method of claim 1, wherein the regrowth layer induces a stress in the substrate.
 6. The method of claim 1, wherein the regrowth layer comprises silicon germanium, and the substrate comprises silicon.
 7. The method of claim 1, wherein the isolation material comprises an oxide.
 8. The method of claim 1, wherein the recess is a trench, the regrowth layer being epitaxially grown on sidewall surfaces and a bottom surface of the trench.
 9. A method comprising: etching a recess in a semiconductor substrate; forming an epitaxial layer on a sidewall of the recess and on a bottom surface of the recess; forming a dielectric material in a remaining portion of the recess; and forming a photo element in the semiconductor substrate proximate the recess, the photo element comprising a p-n junction.
 10. The method of claim 9, wherein the forming the epitaxial layer comprises forming a first portion of the epitaxial layer adjacent the sidewall and the bottom surface, and forming a second portion of the epitaxial layer adjacent the first portion, the forming the first portion not including forming the first portion with a p-type impurity or an n-type impurity, the forming the second portion including forming the second portion with a p-type impurity or an n-type impurity.
 11. The method of claim 9, wherein the epitaxial layer comprises a material different from a material in the sidewall of the recess.
 12. The method of claim 9, wherein the semiconductor substrate comprises silicon, and the epitaxial layer comprises a silicon germanium alloy.
 13. The method of claim 9, wherein the epitaxial layer induces a stress in the semiconductor substrate.
 14. The method of claim 9 further comprising: forming a plurality of inter-metal dielectric layers on a first side of the semiconductor substrate; forming filter on a second side of the semiconductor substrate, the second side being opposite the first side; and forming a lens on the filter.
 15. A method comprising: forming a p-well through a first side of a substrate and in the substrate; forming a recess in the p-well in the substrate; epitaxially growing a regrowth layer in the recess; forming an insulating material in a remaining portion of the recess and on the regrowth layer; forming a photo diode in the substrate and proximate the p-well; forming a plurality of inter-metal dielectric layers on the first side of the substrate; and forming a lens on at least one of (i) at least one of the plurality of inter-metal dielectric layers and (ii) a second side of the substrate, the second side being opposite the first side.
 16. The method of claim 15 further comprising doping the regrowth layer with a p-type or n-type dopant.
 17. The method of claim 15, wherein the epitaxially growing the regrowth layer comprises growing a first portion of the regrowth layer without a dopant and subsequently growing a second portion of the regrowth layer with the dopant.
 18. The method of claim 15, wherein the regrowth layer is a material different from a material of the substrate in a surface of the recess.
 19. The method of claim 15 further comprising thinning the substrate from the second side, the lens being formed on the second side.
 20. The method of claim 15 further comprising forming a color filter on the at least one of (i) the at least one of the plurality of inter-metal dielectric layers and (ii) the second side of the substrate, the lens being formed on the color filter. 